S-parameters of a device-under-test (DUT) may be measured using remote reflectometer heads that are typically connected to a millimeter wave (mmw) vector network analyzer (VNA) by long, flexible coaxial cables that deliver local oscillator (LO) signals and radio frequency (RF) stimulus signals to the multipliers and mixers (down converters) inside the reflectometer heads. The difference in thermal and/or mechanically related phase change in the LO paths used for LO signal distribution between the VNA and the reflectometer heads is a primary contributor to S-parameter measurement instability.
In particular, the flexible high frequency coaxial cables are subject to movement during measurement after system calibration. The movement stresses the dielectric of the flexible coaxial cables, and physically changes the LO paths. The changes lead to impedance changes at points of stress, mainly causing a phase shift to the RF and LO signals reaching the remote reflectometer heads. The phase shift contributes to measurement errors.
In standard 2-port VNA setups, changes to the phases of the RF stimulus signals supplied to the reflectometer heads may not be critical to the accurate measurement of S-parameters. However, phase changes to an LO signal supplied to the respective reflectometer heads in a standard 2-port VNA setup directly leads to measurement instability of the S21 and S12 parameters, because the signal transmitted to and received from the DUT in the respective reflectometer heads will consequently be divided by respective different LO signals. The S11 and S22 parameters are not affected, since the DUT transmit and receive signals are divided in a same reflectometer head by the same LO signal.
Due to frequency multiplication in the reflectometer heads, the phase error imposed on the S-parameters is N-times higher than the phase error imposed on the signals transmitted through the coaxial cables feeding the reflectometer heads, where N is the frequency multiplication factor in the reflectometer heads. The instability issue becomes more severe towards higher frequencies. The amplitude changes induced on the signals transmitted through the coaxial cables due to the movement may have a rather negligible effect due to the limiting nature of the multiplication circuits in the reflectometer heads.
FIG. 1 illustrates a graph of the measured phase error after bending one LO cable of a 2-port VNA with reflectometer heads covering the F-band (90-140 GHz). In multiport VNAs, the LO path phase change problem will occur for any S-parameter measurement in which transmit and receive signals are measured in different heads. In addition to the above described problem with the LO path phase change, in the case of differential setups, phase shifts in RF cables can lead to instability since the RF stimulus phase must be controlled to ensure a known and controllable phase difference between the two signals forming a differential port.
What is needed is cost efficient way of measuring LO path changes and tracking the contribution of the LO path changes to measurement instability, so that the resulting measured change may be applied to update the calibration and remove the effect of the phase change from S-parameter measurements.